The present invention is concerned with control of light emitting diodes (“LEDs”).
The present invention has been developed in response to requirements for aircraft lighting utilising light emitting diodes (LEDs) although it has numerous potential applications in connection with lighting for other purposes. LEDs offer great advantages over more traditional light sources such as filament bulbs. LEDs have a much longer service life than such traditional sources, are more energy efficient and can be chosen to emit only, or largely, in selected frequency ranges. It is known to utilise a bank of LEDs to substitute for a filament bulb e.g. in traffic lights or in external aircraft lighting. Lamps suitable for such purposes are disclosed, for example, in published French patent application FR2586844 (Sofrela S.A.), utilising a PCB bearing a bank of LEDs which together provide the luminous intensity required to replace the filament of a traditional bulb.
It is very well known that a circuit for driving LEDs should incorporate some means for limiting the current passing through them. The resistance of an LED varies with temperature and if no limit is imposed on the current passing through it, the result can be excessive power being dissipated in the LED with consequent damage to it. The simplest current limiter is a resistor in series with the LED. An alternative is to drive the LED (or LEDs) using a constant current source.
A more sophisticated mode of control of LEDs is desirable in certain contexts, aircraft lighting being one example. The lights used at the exterior of an aircraft-navigation lights, landing lights etc.—are required to provide a high level of output optical power and to do so despite large variations in ambient temperature. A simple current control device cannot provide optimal LED performance in this demanding environment.
The use of a microprocessor to control an LED has been proposed in European patent application EP0516398 (Mitsubishi Kasei Corporation). However the intention was to provide a highly stable emission spectrum to serve as a “standard light source”, microprocessor control being used to effect closed loop stabilisation of output wavelength.